This invention relates to demodulating amplitude modulated (AM) signals, particularly radio signals e.g. as used in short range data communication.
When demodulating signals it is sometimes necessary to determine accurately the edges of modulation symbols. For example, a wireless communication system may be configured such that a transmitter transmits AM signals comprising a periodic carrier signal modulated by modulation symbols to a receiver that must demodulate the AM signals in order to recover the information or data contained within the signal. If, for example, the signal conveys digital data comprising a repeating pattern carrier wherein the amplitude varies between two pre-determined levels, the accurate timing of the end of each modulation symbol can be critical to the operation of the system, particularly regarding starting timers that are used to set a starting point for a response at an exact point in time.
When demodulating AM modulation symbols from a modulated signal, it is known to use an envelope detector to extract a signal envelope that conveys the general shape of a modulating signal which, in general, will follow the modulating symbols that generated the modulated signal. Such an envelope detector often comprises a simple rectifier where the output signal of the rectifier is filtered using an analogue filter and then fed to a comparator to provide a demodulated signal.
The analogue filter used in such a solution typically suffers from process variation (differences between devices due to manufacturing processes) which may lead to inaccurate detection of the actual envelope and uncertainties in the timing of modulation symbol edges.
It is also known to use a phase locked loop (PLL) for demodulation in the case where only AM techniques have been used in the modulation of the signal. In such a system, both the modulated signal and a local oscillator signal are fed to a phase detector. When the input signal is modulated by suppressing the carrier, the phase detector will not sense any signal on the reference input and the PLL will lose phase lock. By using a simple lock detector circuit, known per se to a person skilled in the art, demodulation can be achieved.
A disadvantage of using a PLL is that the time that is required to detect phase lock depends upon the oscillator free-running frequency, which itself is typically dependent upon analogue process variation of the analogue components.
The present invention sets out to provide an alternative approach.
From a first aspect, the invention provides a method of demodulating an amplitude modulated radio signal comprising directing said modulated signal to both a phase detector and an edge detector, and using the respective output signals of the phase detector and edge detector to determine an end of a modulation symbol in said signal.
From a second aspect, the invention provides a receiver arranged to demodulate an amplitude modulated radio signal comprising a phase detector and an edge detector that are both arranged to receive the modulated signal, the receiver being arranged to use the respective output signals of the phase detector and edge detector to determine an end of a modulation symbol in said modulated signal.
Thus it will be seen by those skilled in the art that, in accordance with the invention, a receiver directs a modulated radio signal to both a phase detector in order to obtain the envelope of the modulated signal, and an edge detector which can determine an edge in the modulated signal. The outputs from both detectors can be used to determine the end of a modulation symbol and thus provide a demodulated signal output.
Using both a phase detector and an edge detector provides the ability to obtain a demodulated signal that has a reduced uncertainty with regard to the end of modulation symbols when compared to conventional systems without the edge detector.
There are a number of phase detector circuits known per se in the art. In some sets of embodiments, the phase detector comprises a phase locked loop and a lock detector.
In order to determine whether a modulation symbol is present, it is advantageous to convert the input signal, which is conventionally sinusoidal, to a square wave before providing it as an input to the phase detector. In some sets of embodiments, the modulated signal is fed to a sine-to-square converter.
The square wave output referred to above can be used to determine when modulation symbols are present on the modulated signal. In some further sets of embodiments, the output of the sine-to-square converter comprises a square wave when no modulation symbol is present and a constant value when a modulation symbol is present.
It is typically not always known in advance whether the square wave output will comprise a logic high or a logic low value when a modulation signal is present due to the nature of sine-to-square converter circuits. It is common for sine-to-square converters (such as those that utilise Schmitt triggers) to have hysteresis, such that it is not known what value the output will take when the input is below a threshold voltage required to overcome said hysteresis.
The end of modulation symbols present on the modulated signal may be determined by monitoring for a change in the value of the edge detector input from a first constant value to a second value. In some sets of embodiments, the edge detector comprises a first output that latches to a first level when the input to the edge detector is above a first threshold value, and a second output that latches to a second level when the input to the edge detector is below a second threshold value. The first and second thresholds may be the same but typically they are different.
The edge detector could produce an output continuously. In a set of embodiments however it instead monitors for the end of a modulation symbol only once it is known that such a modulation symbol is currently present on the modulated signal. In some sets of embodiments therefore, a combiner is provided which is arranged to reset the edge detector after the phase detector has detected a modulation symbol.
When it is determined that a modulation symbol is present on the modulated signal and the edge detector has been reset, it is then advantageous to monitor for the end of the modulation symbol. Due to the above-mentioned hysteresis effect present on the sine-to-square converter, it is typically not known in advance whether the end of the modulation symbol will be signified by a low-to-high transition or a high-to-low transition, but it is possible to determine the current level of the signal and monitor for a change to the corresponding opposite level. In some sets of embodiments, a combiner is arranged to detect a first output of the edge detector latching to a first output value after resetting the edge detector and monitors for a second output of the edge detector latching to a second output value.
After some time, the modulation symbol will end and the output from the sine-to-square converter will no longer remain constant. In a further set of embodiments, the second output of the edge detector latching to a second output value may be used to indicate the end of the modulation symbol.
In general there is a possibility that errors can occur in any given demodulation scheme and it is advantageous in some circumstances to reduce these errors by validating a detected end of a modulation symbol to ensure that it is genuine. In some sets of embodiments, an edge detected by the edge detector is validated by checking that the edge is followed by the phase detector output going high.
In some sets of embodiments, the demodulator is implemented in a battery powered integrated circuit.
A person skilled in the art would understand that the present invention has applications in a wide range of communication technologies. In some sets of embodiments, the demodulation is provided for use in near field communication (NFC) systems.
A person skilled in the art would also understand that the carrier referred to with regard to the present invention is not limited to a sinusoidal waveform and could comprise other forms including, but not limited to, square waves, saw-tooth waves triangular waves or other periodic waveforms.